[mlir] Allow overriding AbstractDenseDataFlowAnalysis::visitOperation

AbstractDenseDataFlowAnalysis::visitOperation controls how the dataflow
analysis proceeds around control flow. In particular, conservative
assumptions are made about call operations which can prevent some
analysis from succeeding.

The motivating case for this change is https://reviews.llvm.org/D140415,
for which it is correct and necessary for the lattice to be preserved
after call operations.

Some renaming was necessary to avoid confusion with
DenseDataFlowAnalysis::visitOperation.
AbstractDenseDataFlowAnalysis::visitRegionBranchOperation and
DenseDataFlowAnalysis::visitOperationImpl are also made protected
to allow implementation of AbstractDenseDataFlowAnalysis::visitOperation,
although I did not need these to be virtual.

Differential Revision: https://reviews.llvm.org/D140879